An AKR/J murine carcinoma-like tumor was generated in tissue culture from an undifferentiated fibrosarcoma. An autologous immune response was generated against the carcinoma in vivo and in vitro. Subcutaneous injection of low numbers of the carcinoma-like cells resulted in a period of tumor growth, followed by a period of tumor regression. Depression of a number of T-cell functions was observed during the period of tumor growth, whereas T-cell functions returned to normal levels during the regression phase. Cell separation procedures revealed that a population with suppressor cell characteristics directed at T-lymphocyte functions could be identified during the period of tumor growth.